Reconstruction and alignment of vacancies in carbon nanotubes

Vacancies in carbon nanotubes usually aggregate into larger vacancies. Using first-principles and tight-binding calculations, we investigate the alignment of missing atoms and the movement of pentagon-heptagon defects that are formed by reconstructions in large vacancy clusters V-n (n <= 36), where n is the number of missing atoms. In nanotubes with small diameters, missing atoms have a tendency to form a serial network rather than a large hole due to the existence of large curvatures. It is generally found that the parallel alignment of missing atoms along the tube axis is energetically more favorable than the spiral alignment. Thus, the removal of atoms leads to the longitudinal movement of a pentagon-heptagon defect on the tube wall, which is in good agreement with the kink motion observed during superplastic deformation of single-wall nanotubes. The preference of the longitudinal motion of the pentagon-heptagon defect is more prominent in armchair tubes compared with other chiral tubes.